Silicon materials are used in a wide variety of technical applications, including for example, electronic devices, solar cells, chemical sensors, micro-mechanical devices, and electrochemical cells. These and other applications employ different forms of silicon materials, such as single crystal or polycrystalline silicon, amorphous silicon, silicon nanocrystals, porous silicon materials, or thin films of any of these forms of silicon. In many applications, it is useful to be able to modify the chemical or physical properties of the silicon surface according to the function it is to perform. For example, the surfaces of silicon chips are selectively masked prior to doping in the manufacture of electronic devices; and silicon surfaces in chemical sensors may be chemically treated to enhance their performance. In addition to these known modifications of silicon surfaces, new applications may be facilitated by expanding the methods available for processing silicon surfaces. For example, it is desirable to develop methods for treating the surfaces of micro-mechanical devices fabricated from silicon to minimize stiction.
The chemistry necessary to modify silicon depends on the material forming the surface of the silicon material. Since silicon is readily oxidized under ambient conditions, the surfaces of many solid silicon materials include overlayers of SiO.sub.2. Consequently, much of the chemistry of silicon surfaces is largely that of reactions with the SiO.sub.2 overlayer of the oxidized silicon. For example, oxidized silicon reacts with trichlorosilanes, trimethoxysilanes, triethoxysilane, and other silanes having hydrolyzable substituents. Such reactions are of significance to the fiberglass industry and can be used to generate self-assembled monolayers on the oxidized silicon surfaces. The etching of oxidized silicon surfaces to form hydrogen-terminated silicon surfaces is also known.
In addition to the formation of self assembled monolayers on oxidized silicon surfaces, reactions between certain hydrocarbons and clean silicon surfaces have been studied. These reactions are typically carried out with a single crystal silicon surface maintained under ultra high vacuum (UHV) conditions while gas phase reactants such as propylene, ethylene, and acetylene are dosed onto the silicon surface. These reactions typically produce low coverages of small, volatile, Si bonded alkyl chains, and at higher temperatures, SiC. In general, UHV systems are limited to reactions involving smaller hydrocarbon molecules such as those identified above. Further, such gas phase methods may not be suitable for generating self-assembled molecular layers on surfaces because the mechanism by which self-assembly occurs does not necessarily operate effectively in the low density gas phase. There is thus a need for new methods of chemically and physically modifying silicon surfaces by forming molecular layers of any desired size and molecular consistency, both to improve the performance of silicon in present applications and to develop new applications of silicon materials.